Recently, the advancement of high performances in a vehicle, particularly advancement of high performances in passenger car and racing car becomes remarkable, and also it is naturally desired to considerably improve performances of a pneumatic tire to be mounted onto the vehicle in accordance with the above advancement of high performances. Especially, in case of a pneumatic radial tire for passenger car or a pneumatic racing tire, it is necessary and indispensable to considerably improve the steering stability. In this connection, it is generalized to adopt a lower-section profile tire which easily provides a large rigidity in the cornering and is advantageous in the development of a highly improved steering stability, or a tire having a smaller value of a nominal aspect ratio (flattening percentage).
However, as the flattening percentage of the tire becomes smaller, the ground contact width of the treading surface becomes wider, while the ground contact length becomes shorter, so that it is difficult to obtain a uniform ground contact pressure distribution in the widthwise direction of the treading surface and hence the drainage property being high in the dependence upon the ground contact pressure at the ground contact zone of the tire rotating on wet road surface at a higher speed is degraded and it is apt to cause so-called hydroplaning phenomenon at a vehicle speed in a relatively low speed region.
In order to shift the speed of causing the hydroplaning phenomenon to a higher speed size, it is attempted to take various means in the shape and arrangement of grooves in the treading surface of the tread portion most contributing to the improvement of the drainage property, and as a result, the resistance to hydroplaning in the tire having a small flattening percentage is improved to a certain level.
On the other hand, it is simply that the groove shape and groove arrangement define the shape and arrangement of land portions in tread rubber at the treading surface side of the tread portion, which largely affects performances other than the drainage property such as steering stability on dry road surface, resistance to uneven wear and the like. That is, the groove shape and groove arrangement capable of always establishing the drainage property and the other required performances are not necessarily obtained, and there is often caused a case that the groove shape and groove arrangement desired in view of the drainage property are incompatible with the rigidity distribution of land portions required in view of dry steering stability and resistance to uneven wear. In this case, therefore, it is obliged to naturally cause a limit in the degree of improving the drainage property through the groove shape and groove arrangement.
It is, therefore, an object of the invention to provide a pneumatic tire having a tread portion capable of establishing performances such as highly improved dry steering stability and resistance to uneven wear and an excellent resistance to hydroplaning based on a large improvement of drainage property by setting the groove shape and groove arrangement at a large freedom degree including a groove-free case (slick), and particularly pneumatic radial tire for passenger car, pneumatic radial tire for truck and bus or pneumatic racing tire.